what is the impedance of your aftermarket 6x9's... 8 ohm, or 4 ohm, or what...the factory speakers are 8 ohm, if its the factory HU, 8ohm is what its called for.
if you're saying they don't bump as much, it's not because they're underpowered, its because it sounds like your aftermarket's are 4 ohm, and the signals are being cancelled out between the HU and speaker...
you can either match the HU requirements which is an 8 ohm speaker,
or you can get a new HU to match your speakers requirements,
or you can TRY and physically adjust your speakers' impedance (i wouldn't

)